- Email: [email protected]
Microstructural evolution and the role of Pb atmosphere in forming Bi-2223 superconductor
EISEVIER
Physica C 280 ( 1997) 187- 199
Microstructural evolution and the role of Pb atmosphere in forming Bi-2223 superconductor Y.T. Huang apb,L.J. Chen a,* a Department of Materials Science and Engineering, National Tsing Hun University, Hsinchu 300, Taiwan ROC b Materials Research Laboratories, Industrial Technology Research Institute, Churung, Hsinchu 310, Taiwan ROC Received 3 April 1997; accepted 1 May 1997
Abstract The evolution of microstructures during the phase conversion for the Bi(Pb)-Sr-Ca-Cu-0 system has been studied for specimens with cumulative sintering. The changes of surface morphology, crystallinity, composition, weight and diameter were characterized for specimens sintered iteratively at a fixed temperature. Evaporation of Pb from the surface was found to result in a microstructural variation between surface and interior of specimens. The phase conversion for the sintered specimen was significantly enhanced by sir&ring the specimen with fresh powder pellet of the same composition. The enhancement is attributed to the change of Pb atmosphere during sintering treatment. 0 1997 Elsevier Science B.V. Keywords: Bi-2223; Structural phase bansformation; Pb atmosphere; Microstructure
1. Introduction Among the cuprate high-T, materials, the Bi(Pb)Sr-Ca-Cu-0 with a nominal stoichiometry of (Bi,Pb),Sr,Ca,Cu,O,, hereinafter denoted as 2223 phase, was demonstrated to be the most promising superconducting material to carry large electrical current in practical applications [l]. However, it is also known to be very difficult to synthesize the single 2223 phase. Very long sir&ring time was required, usually hundreds of hours, to obtain a nearly pure phase material. Although the discovery of the 2223 (Pb-free) phase was first announced [2] in 1988 and later in the same year the incorporation of Pb was found to
* Corresponding author. Tel.: + 886 3 5718328; fax: + 886 3 5718328; e-mail: [email protected]
significantly speed up the formation [3], the formation mechanisms have not been well understood. A number of mechanisms were proposed to explain the reaction route in forming the 2223 phase. A disproportionation of a 2212 phase into 2223 and 2201 phases was suggested by Nobumasa et al. [4] and Kijima et al. [5]. The 2212 and 2201 phases represent the double Cu-0 layered structure with nominal composition of (Bi,Pb),Sr,Ca&O, and the single Cu-0 layered structure with nominal composition of (Bi,Pb),Sr,Cu ,O,, respectively. Morgan et al. [6] proposed a vapor-liquid-solid-like (VLS-like) growth in which 2223 phase grows by the movement of Bi/Pb-rich droplets. Wang et al. [7] and Bian et al. [8] suggested an intercalation mechanism in which a (Ca,Cu)-rich liquid phase introduces the additional Ca-Cu-0 layer to the 2212 phase and hence forms the 2223 phase. In our previous studies [9,10], the
0921-4534/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO921-4534(97)00447-4
188
Y.T. Huang, LJ. Chen/Physica
occurrence of a transient liquid phase, which is closely related to the formation of 2223 phase, during sintering was investigated. The 2223 phase was proposed to be formed by a nucleation and growth process from the liquid phase. In the present study, the microstructural evolution of bulk specimens during phase conversion were investigated by scanning electron microscopy (SEMI, energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) analySeS.
On the synthesis of high-purity 2223 phase of the Bi(Pb)-Sr-Ca-Cu-0 system, the atmosphere of heat-treatment was found to be one of the dominating factors. Many efforts have been devoted to the studies of phase formation in nitrogen, oxygen, argon and in ambient with different oxygen partial pressure as well as in air [ 1 1- 161. A reduced oxygen partial pressure was found to effectively enhance the efficiency of synthesizing 2223 phase [l l-131. Nevertheless, the atmosphere resulting from the volatile Pb-containing phase of the material did not receive much attention. In the present study the effects of Pb atmosphere on the evolution of microstructures for specimens sintered with and without the Pb atmosphere were investigated.
C280 (1997) 187-199
SEM equipped with a EDAXEDS. For SEM/EDS investigation, the same regions of the specimen were examined repeatedly following heat treatments. XRD patterns of the specimen were taken using a Shimazu XD3A diffractometer with Cu Kol radiation. To study the effect of Pb atmosphere on the formation of 2223 phase, specimens were sintered with fresh pellet samples which were presumed to provide a Pb-rich atmosphere during high temperature. The fresh pellet samples, hereinafter referred to the atmosphere-control (AC) pellet, were prepared with the same starting powder as that for the specimens. The specimens were sintered with and without the presence of AC pellets, cumulatively. As the AC pellets were used for sir&ring, the specimens were placed on the top of the AC pellet and with the surface for characterization facing downward. Since the specimens became a little concave at the surface after the previous experiment, changes in the surface morphology owing to contact with AC pellets was avoided. However, the surface of the specimen was placed face upwards for sintering without the presence of AC pellets. The changes of microstructures, composition, XRD pattern, diameter and weight of specimens were recorded.
2. Experimental 3. Results and discussion The precursor powder with a nominal composition of Bi,,,Pb,.,Sr,,Ca,,Cu,,O, was prepared by a co-precipitation method which was described elsewhere [17]. The calcination of powder was conducted at 760°C for 20 h in ambient atmosphere. Pellet samples with dimensions of 12 mm diameter and N 1.5 mm thickness were prepared by diepressing the calcined powder at 200 MPa. The sintering of specimens was conducted in a tube furnace at 850°C with a heating rate of 180”C/h. The specimens were heat-treated in an ambient atmosphere without gas flowing. After being held at 850°C for a certain period of time, the specimens were cooled down to 800°C at 180”C/h and then quenched in air by quickly being removed from the furnace. The sintering process was repeated until the total sintering time reached 30 h. After each sir&ring process, weight and diameter of specimens were measured. Microstructures were characterized by a CamScan
3.1. Microstructural evolution with time at 850°C Since the Bi(Pb)-Sr-Ca-Cu-0 material contains up to five cations, the phase relations for the formation of desired 2223 phase are known to be very complicated. To investigate the reaction route of phase formation, the characterization of microstructures was often carried out for a series of specimens, e.g. samples sintered at various temperature or time, by XRD, SEM as well as TEM. However, the interaction among grains of various compositions was not directly revealed. As a result, the present experiment was carried out to study the microstructural evolution at a fixed region for the samples heat-treated at a specific temperature for different periods of time. It was aimed to elucidate the interactions among different phases with direct evidence during phase conversion in forming the 2223 superconducting phase.
Y.T. Huang, LJ. Chen/Physica
C280 (1997) 187-199
189
3.1 .I. XRD analysis
To reveal the phase evolution with respect to the sintering time, parts of XRD patterns corresponding to the specimen heat-treated for different periods of time are shown in Fig. 1. Characteristic peaks corresponding to different phases were marked on various patterns for clarity. The major constituents contained in the starting powder were identified to be 2212, Ca,PbO, and CuO phases. After one hour sintering, the corresponding peaks of CuO phase disappeared. The amount of Ca,PbO, decreased as the cumulative sintering time (hereinafter denoted as t,) increased and became undetectable as tf > 4 h. The 2212 phase remained and was the most prominent phase after 4 h sintering. The 2223 phase emerged at t, = 4 h as inferred by the appearance of the diffraction peaks of (0010) and (115) at 28= 23.9” and 26.2”, respectively, The intensities of diffraction peaks corresponding to the 2223 phase become stronger as tc increased. The peaks revealing the simultaneous presence of 2212 and 2223 phases are marked on the pattern for t, = 20 h. Since peaks corresponding to the two phases are appreciably
powder *
15
20
25
30
35
40
45
50
55
62 8
0.5 -
&,
0.4..
$
0.3 -
l**
l
l
l
l l
F 9
0.2 -
0” z
0.1 -
+ 0.0
**
0
5
IO
Cumulative
I5
20
25
30
sintering time, TV(h)
Fig. 2. XRD peak intensity ratio, H(oO~)/H(oO~) specimen at various sintcring time.
+ L@O8),of a
overlapped, it is convenient to use the change of peak-intensity ratio of H(OO10)/H(OO10) + L(OO8) to represent the progress of ph%e convezion. H and L stand for the diffraction peaks corresponding to the 2223 and 2212 phases, respectively. As shown in Fig. 2, the phase conversion was quite rapid at first and slowed down for tc longer than 10 h. The change of conversion rate at long tc will be discussed later. It is well documented [5,9,18-201 that the Ca,PbO, is one of the key constituents to result in a liquid phase which accelerates the growth of 2223 phase in the Bi(Pb)-Sr-Ca-Cu-0 system. As revealed in Fig. 1, the 2223 phase was absent from the XRD pattern in specimens with t, up to 2 h. During the same period, Ca,PbO, and CuO diminished significantly with time. There is a time lag between the dissolution of Ca,PbO, phase and the formation of 2223 phase. It is attributed to the small quantity of the formed 2223 phase that is difficult to be detected by XRD. The peaks corresponding to the Ca,PbO, phase disappeared from XRD patterns for tc > 4 h. However, the phase conversion still proceeded. It is suggested that most of the Ca,PbO, dissolves into a liquid phase and acts as a source to provide Ca and Pb atoms in growing the 2223 phase.
20 degree (CuKa) Fig. 1. XRD patterns of starting powder and a specimen sintered with various cumulative time. 0: 2212, 0: 2223, *: Ca,PbO,, V: CuO, l : (Ca,Sr),CuO,, 0: (Ca,Sr),,Cu,,O,,.
3.1.2. Weight loss and diameter change Fig. 3 shows the weight and diameter changes of the specimens with respect to the sintering time. The
Y.T. Huang. LJ. Chen/Physica
weight of specimens decreased significantly during the first hour. It is attributed mainly to the removal of moisture absorbed in the starting powder. As f, increased the weight loss continued but became sluggish. Evaporation of specific chemical species such as PbO is proposed to be responsible for the weight loss, and/or the change of oxygen stoichiometry when Pb+4 reduced to Pb+’ as it was incorporated into 2223 phase from the parent phase of Ca,PbO, [21]. The diameter of the specimen also showed a great shrinkage during the first hour. It is believed that the formation of liquid phase by dissolving 2212, Ca,PbO,, CuO and/or other intermediate phases is responsible for this dimensional change. In the initial stage of reaction, the constituents responsible for the occurrence of liquid phase are finely dispersed in the powder and the formation of liquid phase is therefore prominent. The great shrinkage of specimen can be expected due to the elimination of porosity and rearrangement of grains whenever abundant liquid phase is present. However, the specimen started to swell as tc exceeded 2 h and continued as t, increased. As predicted, if the reaction follows a liquid-phase-sintering mechanism [22], the specimen swells if that liquid phase is absorbed by the solid phase. It is consistent with our previous suggestion that the 2223 phase nucleated and grew up from the liquid phase [lo]. The starting constituents of 2212, Ca,PbO, and CuO dissolved into a liquid phase which provided the chemical species for growing 2223 phase. The 2223 grains grew up anisotropically with plate-like shape and left a lot of voids among them. It is the dominating mechanism of the diameter change for r, > 2 h. 3.1.3. SEM/ EDS analyses Characteristic microstructural evolution of surface morphology with sintering time is shown in Fig. 4. Since the back-scattered electron image can reveal compositional contrast of impurity phases and their distribution, it was chosen for obtaining the microstructural images. Fig. 4(a) shows the starting microstructure prior to the heat treatment where the sizes of most grains ranged from 0.3 to 1.0 km. The grains exhibited three typical contrasts of black, dark-gray and bright. The areas with black contrast correspond to the locations of pores. The compositions of specific areas or grains were determined by
C280 (1997) 187-199
EDS. Because the typical sampling depth of EDS is about 1 km, it is difficult to determine precisely the composition of individual small grains. However, the dark-gray phase was always found to contain much more Cu and Ca elements than the bright phase. For some selected large grains or agglomerates (m 1 pm> of bright phase, the average composition was determined with the cation ratio of Bi:Pb:Sr:Ca:Cu = 1.940.11: 1.95:0.99:2.0. The ratio corresponds closely to the 2212 phase. The surface morphology changed drastically for specimens subjected to heat-treatment for 1 h. The compositional contrast between the bright and darkgray phases became more evident. The fraction of the dark-gray phase is about 10% as estimated from the micrograph. As r, increased, large plate-like grains were found in specimens with r, = 4 h. Some of the large plate-like grains viewed edge-on are seen clearly in Fig. 4(d). Compositional analysis by EDS showed that they correspond to the 2223 phase. By comparing the same region in micrographs corresponding to fc = 2 and 4 h, most of the observed 2223 grains in specimen with rc = 4 h were found to grow from those embryos found in specimens with tc = 2 h. This indicated that the 2223 grains were mostly nucleated prior to f, = 2 h. However, the presence of 2223 phase was still undetected by XRD owing to its small quantity in specimen with tc = 2 h. For t, = 4 h, the anisotropy of 2223 grains became such that the aspect ratio of broad face to
0.0
8
0
&
0
$$ -1.6 00 . & -1.7 8 9 -*.* ..6”On
0
00 0
%j :;:; :
00 0
3
iii 2 8
“4 6 .
0
-2.2 :
,g
. -0
0
qo0
0 -2.1 -
o”
.
-4
Cl
CD
_ 4 ,i? 0 -
000 - -12
-2.3
’ 0
.
’ 5
.
’ 10
’ I5
.
’ 20
.
’ 25
.
’ 30
Cumulative sintering time, t:(h) Fig. 3. Weight aud diameter changes of a specimen with respect to cumulative sinteriug time.
,lOpm, Fig. 4. Surfam morphology
of a specimen cumulatively
sintered for (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 8 h, (0
192
Y.T. Huang, LJ. Chen/Physica
Table 1 Compositional analyses of specific phases and starting powder by EDS and ICP-AES, respectively
Bi 2212 2223 1424’ 2:lb Ca, PbO, Bullc(1, = 4 h) Powder(by ICP-ASS)
Pb
Sr
24.5 2.9 21.8 21.9 2.9 19.2 1.8 0.3 13.6 3.7 0.7 6.5 9.8 7.8 22.9 17.9 2.5 17.9 17.80 4.22 19.44
Ca
Cu
15.7 23.1 21.9 54.3 48.6 24.8 23.59
33.3 33.2 62.5 34.7 10.9 36.7 34.95
4b Alkaline earth cuperateswith fonuula of (Ca,Sr, _x),4Cu2404, aud (Ca,Sr, _,),CuO,, respectively. can reach 30. The typical thickness of grains is less than 0.3 pm as observed for grains viewed edge-on. The broad face of grain is commonly known to correspond to the ub plane. Compositional analysis of specific grains in specimen with f, = 2 and 4 h was conducted by EDS. The results are listed in Table 1. The small flaky grains with bright contrast correspond to the 2212 phase, and the thin plate-like grains with similar contrast but much larger aspect ratio correspond to the 2223 phase. In addition to the distinct appearance, the Sr/Ca ratio and concentration of Bi were always lower in the 2223 phase than those in the 2212 phase. For 2212 phase the Sr/Ca ratios ranged from 1.3 to 1.5, whereas they were 0.8 - 0.9 for the 2223 phase. The Bi concentration was found to be 23.5 25.5 at% in the 2212 phase and usually less than 22.0 at% in the 2223 phase. It is worthwhile to note that the Pb concentration is higher and the Sr/Ca ratio is lower for the 2212 grains in the heat-treated specimens than those in the as-pressed specimens as mentioned previously. This may be attributed to the existence of a thin layer of liquid phase rich in Pb, Ca and Cu atoms which formed on the surface of these grains during phase conversion. The TEM study of amorphous layers between grains was previously reported [ 19,231. thickness
ent
For the grains with dark-gray phases were identified.
contrast, two differThe first one is
(Ca,Sr,_X),,Cu2404, with x - 0.6 and the other is (Car&, _,),CuOs with y - 0.9, hereinafter denoted as 14:24 and 2:l phase, respectively. These two phases are similar in appearance but with a slight difference in contrast. The 2:l phase is of darker appearance than the other one, which is consistent
C 280 (1997) 187-199
with the fact that back-scattered electrons are lower
in intensity for the smaller mean atomic number in the 2:l phase. Since these two dark-gray phases are relatively small in quantity and the corresponding XRD peaks overlapped with those of 2212 phase, it was difficult to identify them unambiguously from XRD patterns. For the XRD pattern with fc = 2 h, as shown in Fig. 1, the first three strongest peaks belonging to the two dark-gray phases are marked following that reported by Reardon and Hubbard [24]. However, only the peak of 2: 1 phase at 28 36.3” can be unambiguously identified when compared with the peaks of 2212 phase shown above. In Fig. 4(b)-(h), most of the dark-gray grains were firstly found in the specimens with rc = 1 h, i.e. in Fig. 4(b). The grain sizes of dark-gray phases are about several pm as seen in Fig. 4(b), which is evidently larger than the average grain size of starting powder as shown in Fig. 4(a). It is suggested that these large dark-gray phases are formed by the phase-conversion reaction. This is consistent with XRD results, in which no corresponding peaks of both 1424 and 2:l phases were present in the starting powder but they showed up in specimens for r, > 1 h. The fifth phase seen from the image with r, = 4 h are sphere-like grains, bright in contrast, and 0.3 - 0.5 pm in diameter. It is difficult to determine the exact composition by EDS. However, these grains always possess a unique Ca/Pb ratio close to 2/l and much higher Pb concentration with respect to other phases. They are thus inferred to be the Ca,PbO, grains. Typical compositions of this species are listed in Table 1. The average compositions of large areas (120 pm X 90 pm) for the specimens with t, = 4 h determined by EDS and starting powder analyzed by an inductively coupled plasmaatomic emission spectroscopy (ICP-AES) method are also listed in Table 1 for comparison. For t, exceeding 8 h, some 2223 grains grew to impinge one another. For f, = 12 h, most of the 2223 grains were linked together and stopped growing. Since the 2223 phase grains tend to grow anisotropitally or edgewise, the impediment of growing front slowed down the growth rate of 2223 phase. Nevertheless, small new 2223 grains emerged and grew up quickly before they touched neighboring grains of the same phase. During the phase conversion, the flaky grains of 2212 phase were always found to
193
Y.T. Huang, LJ. Chen/ Physica C 280 (1997) 187-199
decrease in size with f,. This indicated that the dominating mechanism of forming the 2223 phase is not the intercalation mechanism for our specimens; otherwise, the 2212 grains would instead grow larger and increase concentrations of both Ca and Cu elements. The compositions of specific 2212 grains analyzed by EDS showed no significant change with increasing t,. For the specific circled area A in r, = 8 h micrograph, the large grain at the bottom is a 2223 grain. The small bright grains distributed on it are of 2212 phase. The dark-gray grain on left side is a 1424 grain. The needle-like grains present neighboring the bottom grain are also of 2223 phase. The 2212 grains diminished as t, increased. Although they look like being isolated above the 2223 grains in the beginning, they may change their size through surface diffusion in transporting mass during phase conversion. The presence of thin-layer liquid phase, observed in previous TEM studies [19,23], along the grain boundary is likely to enhance the mass transport. This scenario is the possible mechanism of phase conversion at long f,. The 14:24 grains also decreased in size but only very slightly. The bottom 2223 grain evidently stretched out downward with increasing r, up to 12 h, but the growth became sluggish as r, increased further, since it was impeded by the surrounding 2223 grains. In the circled area B, there were either two 2223 grains on both sides of a 2:l grain or one large 2223 grain beneath the 2: 1 grain. The 2223 grain finally protruded into the 2: 1 grain as phase conversion proceeded. Similarly, the 2223 grain cut a 2212 grain apart at the circled area C as t, increased. It is suggested that the growth front of the 2223 grain is very active and may dissolve the other phases catalytically. From the present observation, it is clearly shown that the growth front stretched out very quickly when it was not impeded by other growing 2223 grains. The proceeding of the growth front stopped when it was in contact with other grains of the same phase. From these observations, it is believed that the phase conversion of the 2223 is actually rather rapid in contrast to the previous understanding of sluggishness. It is suggested that the highly anisotropic growth results in the sluggishness and difficulty in synthesizing the phase in pure form. Based on this consideration, the efficiency of synthesizing the 2223 phase
O> 0
5
10
15
20
25
30
Cumulative sintering time, k (h) Fig. 5. Chemical composition determined by EDS analysis for specimens at various sintering time.
can be substantially enhanced by taking an intermediate grinding process during the sintering treatment. The grinding process is expected to break up the interconnected 2223 grains and enhance the conversion rate by creating new growth fronts. Fig. 5 shows the compositional change of the examined surface for a large area (120 X 90 pm) with increasing r,. No significant changes in compositions of elements, except Pb, were found. It is clearly seen that the concentration of Pb dropped significantly between tc = 1 and 2 h. This coincides with the change of Ca,PbO, revealed in the XRD patterns shown in Fig. 1. It is attributed to the great dissolution of Ca2Pb0, in the initial stage of phase conversion which resulted in the substantial loss of Pb by evaporation. Comparing the results shown in Figs. 2 and 3, and the Pb loss in Fig. 5, the results appear to be inconsistent in depicting the phase conversion. The significant Pb-loss for specimens characterized by EDS appeared at t, = 2 h. Significant slowing down of the formation of 2223 phase was found at r, > 10 h as revealed by XRD. The changes of weight and diameter of the specimens were relatively gradual and smooth. The discrepancy is believed to result from different spatial resolution for respective measurements. The diameter and weight changes of specimen represent the overall variation of specimen during phase conversion. The XRD analysis revealed the changes in microstructures within several tens of pm from the surface.
194
Y.T. Huang, LJ. Chen/Physica
The EDS analysis obtained compositional information from the region a few pm in depth. Since the evaporation of Pb is much easier from material near the surface of specimen, there should be a concentration variation along the depth of specimen. The results suggested that the microstructures in the interior of bulk specimens are different from those of the surface. For characterizations conducted at different depths of specimens, different results were obtained. 3.2. Effects of Pb atmosphere on microstructural evolution In the previous part of study, the phase conversion of specimen was found to vary from the surface to the interior for bulk specimen due to the inhomogeneous evaporation of Pb (hereinafter stands for the rich Pb-containing species). In addition, the formation of 2223 phase in the previous part of study as revealed by XRD analysis is much less than expected for total sintering time up to 30 h. The phase conversion rate was found to be much higher for a sample heat-treated with normal sintering up to 30 h, where the sample was heat-treated at a fixed temperature for a certain period without interruption. It was suggested that the significant loss of Pb for the cumulative sintering process may be responsible for the low conversion rate. A study of the Pb atmosphere for the formation of 2223 phase on the microstructural evolution has been carried out. 3.2.1. XRD Analysis To illustrate the differences between the cumulative and the normal sintering, a series of samples for normal sintering were prepared of which each sample was heat-treated at 850°C for a certain period of time. The starting powder of the series of samples is the same as that used for cumulatively sintered specimens. Pellet samples were prepared and sintered at 850°C for 2 N 40 h. Fig. 6 shows the H(0010)/[H(0010) + L(OOS)] ratio with respect to sintezg time fz both processes. It is clearly seen that the phase conversion is much faster for the samples sintered under normal processing for sintering time within 30 h. The possible mechanism to cause this difference will be discussed later. For the specimens sintered cumulatively with the
C 280 (1997) 187-199
1.0 -
s 8 =i
normal
.
sAlterin%
0.8 -
0
o,”
I * I. 10
20
30
q
n
0
,
‘---
0.0 - I$
n
0
Pb-atmosphere
I. 40
I. 50
Sintering time
I. 60
exp.
I.
1. 60
70
I 90
(h)
Fig. 6. XRD peak intensity ratio, H@O~)/H(OO1O)+ L@O8), with respect to sinteriag time for normal and cumulative sinterhg, respectively. Filled symbols denote specimens sintered with AC pellets.
AC pellets for additional two 10-h periods, i.e. t, = 40 and 50 h, the phase conversion was found to be significantly enhanced. The 2212 phase almost disappeared on the XRD pattern with t, = 50 h. Little change was recognized by XRD analysis for higher t, of 60, 70 and 90 h in spite of sintering with or without AC pellet. This indicates that the phase conversion of specimens was nearly completed for t, exceeding 50 h. XRD patterns of specimens at various tC are shown in Fig. 7.
15
20
25
30
35
40
45
50
55
29 degree (CuKa) Fig. 7. XRD patterns of specimen at various t,. Asterisks denote specimens sintered with AC pellets. (0: 22,2, 0~ 2223, +: (CaM,CuOJ.
Y.T. Huang, LJ. Chen/Physica
,
IS - 12
.
0
B
.z
.
”
8 0
0 -2.7 -
0.
00
-2.4 -
3
0
-
.
e-6
0
10
20
30
4U
50
6U
70
80
s --9
Pb-almosphererxp.
-
0
_(D
0.
1
90
-
-12
Cumulative sintering time, tc (h) Fig. 8. Weight and diameter changes of specimens with respect to sintering time. Filled symbols denote specimens sintered with AC pellets.
3.2.2. Diameter and weight changes Fig. 8 shows the changes of diameter and weight for specimens processed by cumulative sintering. The filled and hollow data marks stand for the specimens sintered with and without AC pellets, respectively. The results for t, within 30 h are also presented for comparison. As described in an earlier section, the diameter and weight changes appeared to be saturated for t, approaching 30 h. However, specimen diameter was found to continue increasing for specimens sintered under the effect of Pb atmosphere. The change of diameter reached a saturation region for tc exceeding 50 h. The. results are consistent with those obtained by XRD analysis as discussed earlier. The swelling of specimen was attributed to the same liquid-phase-sintering mechanism. Nevertheless, the weight loss of specimens was found to continue appreciably with t, and did not reach a saturation point. The mechanism of the continued loss in weight is not well understood at this time. 3.2.3. SEM/ EDS characterization The compositional changes in a large area ( w 120 x 90 p,m) of specimens are shown in Fig. 9. The compositional changes of specimens at t, within 30 h are also illustrated for comparison. It is clearly seen that only the concentration of Pb varied significantly in the presence of AC pellets. For specimen sintered with AC pellets, the Pb concentration was found to increase as shown in Fig. 9. Further sinter-
C 280 (1997) 187-199
195
ing without the presence of AC pellet, at tc = 60 h, the Pb concentration decreased significantly. The same results were reproduced for sir&ring with and without the presence of AC pellets for t, = 70 and 90 h, respectively. It is suggested that the Pb-rich species dissipate mostly from AC pellets, in which the Ca,PbO, phase is abundant, in establishing the equilibrium partial pressure of Pb during sintering. As a result, the Pb concentration of specimens increased as Pb species condensed on and/or absorbed by the specimens during cooling period, especially near the surface region of specimens. However, for sintering without the AC pellets, the Pb in specimens evaporated in order to balance the partial pressure during high temperature. Therefore, the Pb concentration was found to decrease in specimens sintered without the presence of AC pellets. The suggested mechanism explains well the difference between the normal sintering and the cumulative sintering in phase conversion rate as shown in Fig. 6 with tc under 30 h. The equilibrium partial pressure of Pb was established in the ambient by evaporating Pb from specimens whenever they were heated up to 850°C and some of Pb was dissipated and condensed onto the wall of the quartz tube as confirmed by the appearance of a layer with light-yellow color. For cumulative sir&ring process, the Pb atmosphere was established and some of Pb dissipated away during each
i0
"'.'.."'.'. 0 IO 20
30
Pb-atmosphereexp. 40
50
I.I.I.1 60 70 80
90
Sintcring time, tc (h) Fig. 9. Chemical composition determined by EDS analysis for specimens at various sintering time. The filled marks denote specimens sintered with AC pellets.
1%
Y.T. Huang, LJ. Chen/Physica
sintering. Therefore, each heating-cooling cycle results in a certain loss of Pb concentration from the specimens. Since Pb-containing phase (e.g. Ca,PbO,) is the key constituent in forming liquid phase, the significant loss of Pb leads to the slow conversion rate of forming 2223 phase. For the normal sintering process, the equilibrium Pb atmosphere was established as the specimen was heated up to the sintering temperature and persisted during the sintering period. Although the dissipation of Pb from the specimen still occurs, the rate is substantially lower compared with that of iterative heating-cooling cy-
C280 (1997) 187-199
cles for cumulative sintering process. As a result, the Pb concentrations in the specimens sintered differently were found to vary. It is worthwhile to mention that, in spite of the difference in phase conversion rate, the species of crystalline phases present in samples of the two processes are the same. The amount and grain size of the phases were found to vary with process. Fig. 10 shows a series of micrographs revealing the evolution of microstructures. The small flaky 2212 grains seen in the micrograph with r, = 30 h diminished significantly as the specimens were sin-
Fig. 10. Surface morphology of specimens at r, = (a) 30 h, (b) 40 h, (c) 50 h, (d) 60 h, (e) 70 h and (f) 90 h.
Y.T. Huang, L.J. Chen/Physica Table 2 Compositional
analyses
of specific grains at various
Grain
r, (h)
Bi
Pb
2:1 a
30 40 ’ 50 ’ 60 70 ’ 90
3.8 2.3 2.4 3.5 3.0 3.2
0.5 5.6 3.8 1.0 8.7 0.5
30 70 c 90
1.8 1.7 1.8
0.1 0.2 0.0
14~24~
Sr
r,
Ca
Cu
6.6 5.2 5.8 6.8 5.5 6.8
54.4 53.9 52.5 52.0 47.7 53.5
34.7 33.0 34.5 36.6 35.1 36.0
13.4 13.0 13.0
21.9 21.9 22.4
62.8 63.2 62.8
‘b Alkaline earth cuperates with formula of (Ca,Sr, _&ZU~~O~, and (Ca,Sr, _,),CuO,, respectively. ’ Specimens sir&ted in the presence of AC pellets.
tered further for 10 h ( f, = 40 h) in the presence of AC pellets. It is consistent with the result of XRD analysis, in which the diffraction peaks of 2212 phase decreased notably at f, = 40 h. As tc increased to 50 h, the small 2212 grains almost completely disappeared as revealed by the corresponding micrographs. The surface morphology did not change markedly as t, was increased further. The observation is consistent with the XRD analysis which reveals that the phase conversion entered a saturation region for rc exceeding 50 h. Slight but significant changes were observed for the dark-gray phases. For the grains marked A in the micrograph with re = 30 h, they were identified to be 2: 1 phase. Their appearance changed into a fractured surface morphology and bright in contrast when they were sintered with AC pellet. However, the same regions were converted back to dark-gray regions with smooth appearance for the specimen sintered without AC pellets at f, = 60 h. Similar results were observed for f, = 70 and 90 h. For the circled 2: 1 phase grain at the middle-left of Fig. lo(a), Table 2 lists the change of chemical composition with respect to sintering time. From compositional analysis, the 2:l phase grain with bright contrast contained higher concentration of Pb than that with dark-gray contrast. The other elements, however, did not vary significantly in comparison. The 2:l phase seems to be less stable in the presence of rich Pb atmosphere when sintered with AC pellets. They interacted with Pb atmosphere and formed a phase with fractured appearance with high Pb concentration. The grains
C 280 (1997) 187-199
197
became smooth in appearance and dark-gray contrast again when they were heat-treated further in the absence of AC pellets. For each cycle of sintering with and without the presence of AC pellets, the grain sixes of 2:l grains were found to decrease significantly. The Pb atmosphere established with the aid of the AC pellets may speed up the dissolution of the 2:l phase and thus provide the source of Ca and Cu elements for further phase conversion of the remaining 2212 grains. The dissolution of the 2:l phase in the presence of the Pb atmosphere may also promote the formation of an additional amount of liquid phase in the specimens and further enhance the phase conversion as observed for the specimens sintered for rc = 40 and 50 h. This mechanism is consistent with the previous suggestion that the significant depletion of Pb concentration in specimen may result in the retardation of phase conversion for cumulative sin&ring process at rc below 30 h. In addition, as shown in Fig. 7, the diffraction peak of 2: 1 phase at 2 0 N 36.3” for specimens with rc = 30 h was found to be absent in the specimens sir&red with fe exceeding 40 h. The B grains were identified to be 1424 phase. For some remaining 14:24 phase grains found at various r,, the chemical compositions determined by EDS are listed in Table 2. It is clearly seen that the Pb concentration was slightly increased and the concentration of other elements maintained the same ratios for specimens sintered with AC pellets. In addition, the 1424 grains as seen in micrographs always showed the same dark-gray contrast despite the presence of AC pellets for sintering. It seems that this phase did not change with the effect of Pb atmosphere. The slight increase of Pb concentration for these grains is likely to result from a simple condensation of Pb-rich phase from the atmosphere for sintering with AC pellet rather than an absorption by chemical reaction. For the grain marked C in the specimen with ?, = 50 h, it changed its appearance from a grain with bright contrast, which is recognized as a 2212 grain, into a rounded grain with dark-gray contrast. This grain did not appear before t, = 40 h. The rounded dark-gray grain was identified to be of a typical 1424 grain. The observation may help to clarify the formation route of 1424 phase grains. During phase conversion, the 2212 grains decom-
Y.T. Huang, LJ. Chen/Physica
posed when reacted with other constituents in forming the liquid phase and the resulted 2223 phase. As the phase conversion reaction proceeds disproportionally, where the other essential reactants for reaction are insufficient to consume the whole 2212 grain, the remaining chemical species are very likely to form the 1424 phase. The present observation supports the earlier suggestion that the 14:24 grain phase may be formed during phase conversion, since the grain size of the dark-gray phase is much larger than that of starting powder. In the same context, the formation of 2:l phase possibly resulted from another disproportional reaction of the constituents in the starting powder such as Ca,PbO,, because the 21 phase is closely related to the presence of Pb atmosphere. The 2:l phase may be generated from the liquid phase, which is rich in Ca, Pb and Cu, by simply depleting Pb. This suggestion is strongly supported by the recent finding by Sung and HellStrom [25], in which the 2:l phase was found to form near the interface of Ca, PbO, + CuO/2212 during phase conversion. The present study indicates that the Ca,PbO, phase decomposes readily at the initial stage of phase conversion and the 2:l phase is not stable in the Pb-rich atmosphere during sit&ring. The 2: 1 phase grains were found to decrease with sintering time and no new 2:l grains were found to form in the areas examined in the present study for f, exceeding 4 h. The impurity phase of more interest in obtaining high-purity 2223 phase is the 1424 phase. It is commonly found in the 2223 bulk and tape-wire materials. This phase may be formed by the decomposition of the 2212 phase at any stage when the disproportional reaction occurs during the phase conversion.
4. Conclusions The interactions among various intermediate phases during phase evolution in forming the 2223 phase were elucidated with SEM/EDS and XRD analyses. The results clearly indicated that the dominating mechanism in forming 2223 phase goes through the nucleation and growth route. The strongly anisotropic growth of 2223 phase grains was sug-
C 280 (1997) 187-199
gested to be responsible for the retardation in synthesizing the 2223 phase in pure form. Evaporation of Pb-containing species from the surface is suggested to result in an inhomogeneous microstructure from the surface to the interior of bulk specimens. By a simple method to vary the condition of Pb atmosphere for sintering, the phase conversion reaction of 2223 phase was found to be significantly varied. For the specimens of which the Pb concentration was significantly depleted, the phase conversion was found to be very sluggish or even stopped. By supplying an appropriate Pb atmosphere, the conversion reaction was found to proceed significantly faster. In addition, the presence of the alkaline-earth cuprate phases was also found to be closely related to the condition of Pb atmosphere during sintering treatment. These results provide important insight in synthesizing high-purity 2223 phase for the Bi(Pb)-Sr-Ca-Cu-0 system. References [l] A.P. Malozemoff, Q. Li, S. Fleshier. G.N. Riley Jr., G. Sbnitchler. D. Aizcd, Chin. J. Phys. 34 (1996) 222. [2] H. Maeda, Y. Tanaka, M. Fukutomi, T. Asano, Jpn. J. Appl. Phys. 27 (1988) L209. [3] M. Takano, J. Takada, K. Gda, H. Kitaguchi, Y. Miura, Y. Ikeda, Y. Tomii. H. Ma&i, Jpn. J. Appl. Phys. 27 (1988) LlO41. [4] H. Nobumasa, K. Shim& Yukishige, Y. Kitano, T. Kawai. Jpn. J. Appl. Phys. 27 (1988) L846. [5] N. Kijima, H. Endo, J. Tsuchiya, A. Sumiyama, M. Mizuno, Y. Oguri. Jpn. J. Appl. Phys. 27 (1988) Ll852. [6] P.E.D. Morgan, R.M. Housley, J.R. Porter, J.J. Ratto, Physica C 176 (1991) 279. [7] M. Wang, Cl. Xiong, X. Tang, Z. Hong, Physica C 210 (1993) 413. [8] W. Bian, Y. Zhu, Y.L. Wang, M. Suenaga, Physica C 248 (1995) 119. [9] Y.T. Huang, W.N. Wang, S.F. Wu, C.Y. Shei, W.M. Humg, W.H. Lee, P.T. Wu, J. Am. Ceram. Sot. 73 (1990) 3507. [lo] Y.T. Huang, C.Y. Shei, W.N. Wang, C.K. Chiang. W.H. Lee, Physica C 169 (1990) 76. [l 11 U. Endo, S. Koyama, T. Kawai, Jpn. J. Appl. Phys. 27 (1988) L1476. [12] S. Koyama, U. Endo, T. Kawai, Jpn. J. Appl. Phys. 27 (1988) Ll861. [13] K. Aoto. H. Hattori, T. Hatano. K. Nakamura, K. Ogawa, Jpn. J. Appl. Phys. 28 (1989) L21%. [14] N.M. Hwang, G.W. Bahng, H.G. Moon, J.C. Park, Appl. Phys. Lett. 54 (1989) 1588. [15] Y.L. Chen, R. Stevens, J. Am. Ceram. Sot. 75 (1992) 1160. [16] W. Zhu, P.S. Nicholson, J. Appl. Phys. 73 (1993) 8423.
Y.T. Huang. LJ. Chen/Physica [17] D.H. Chen. C.Y. Shei. S.R. Sheen, CT. Chang, Jpn. J. Appl. Phys. 30 (1991) 1198. 1181 T. Uzumaki, K. Yamanaka, N. Kamehara, K. Niwa, Jpn. J. Appl. Phys. 28 (1989) L75. [19] Y.L. Chen, R. Stevens, J. Am. Ceram. Sot. 75 (1992) 1142. [20] W. Wang-Ng, C.K. Chiang, S.W. F&man, L.P. Cook, M.D. Hill, Am. Ceram. Sot. Bull. 71 (1992) 1261.
C 280 (1997) 187-199
199
[21] M.G. Smith, D.S. Phillips, D.E. Peterson, J.O. Willis, Physica C 224 (1994) 168. [221 R.M. German, Liquid Phase Sintering, ch. 4, Plenum Press, New York, 1985. p. 65. [231 H.K. Liu, R.K. Wang, S.X. Dou, Physica C 229 (1994) 39. [24] B.J. Reardon, G.R. Hubbard, Pow. Diff. 7 (1992) 142. [25] Y.S. Sung, E.E. Hellstrom, Physica C 253 (1995) 79.
Physica C 280 ( 1997) 187- 199
Microstructural evolution and the role of Pb atmosphere in forming Bi-2223 superconductor Y.T. Huang apb,L.J. Chen a,* a Department of Materials Science and Engineering, National Tsing Hun University, Hsinchu 300, Taiwan ROC b Materials Research Laboratories, Industrial Technology Research Institute, Churung, Hsinchu 310, Taiwan ROC Received 3 April 1997; accepted 1 May 1997
Abstract The evolution of microstructures during the phase conversion for the Bi(Pb)-Sr-Ca-Cu-0 system has been studied for specimens with cumulative sintering. The changes of surface morphology, crystallinity, composition, weight and diameter were characterized for specimens sintered iteratively at a fixed temperature. Evaporation of Pb from the surface was found to result in a microstructural variation between surface and interior of specimens. The phase conversion for the sintered specimen was significantly enhanced by sir&ring the specimen with fresh powder pellet of the same composition. The enhancement is attributed to the change of Pb atmosphere during sintering treatment. 0 1997 Elsevier Science B.V. Keywords: Bi-2223; Structural phase bansformation; Pb atmosphere; Microstructure
1. Introduction Among the cuprate high-T, materials, the Bi(Pb)Sr-Ca-Cu-0 with a nominal stoichiometry of (Bi,Pb),Sr,Ca,Cu,O,, hereinafter denoted as 2223 phase, was demonstrated to be the most promising superconducting material to carry large electrical current in practical applications [l]. However, it is also known to be very difficult to synthesize the single 2223 phase. Very long sir&ring time was required, usually hundreds of hours, to obtain a nearly pure phase material. Although the discovery of the 2223 (Pb-free) phase was first announced [2] in 1988 and later in the same year the incorporation of Pb was found to
* Corresponding author. Tel.: + 886 3 5718328; fax: + 886 3 5718328; e-mail: [email protected]
significantly speed up the formation [3], the formation mechanisms have not been well understood. A number of mechanisms were proposed to explain the reaction route in forming the 2223 phase. A disproportionation of a 2212 phase into 2223 and 2201 phases was suggested by Nobumasa et al. [4] and Kijima et al. [5]. The 2212 and 2201 phases represent the double Cu-0 layered structure with nominal composition of (Bi,Pb),Sr,Ca&O, and the single Cu-0 layered structure with nominal composition of (Bi,Pb),Sr,Cu ,O,, respectively. Morgan et al. [6] proposed a vapor-liquid-solid-like (VLS-like) growth in which 2223 phase grows by the movement of Bi/Pb-rich droplets. Wang et al. [7] and Bian et al. [8] suggested an intercalation mechanism in which a (Ca,Cu)-rich liquid phase introduces the additional Ca-Cu-0 layer to the 2212 phase and hence forms the 2223 phase. In our previous studies [9,10], the
0921-4534/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO921-4534(97)00447-4
188
Y.T. Huang, LJ. Chen/Physica
occurrence of a transient liquid phase, which is closely related to the formation of 2223 phase, during sintering was investigated. The 2223 phase was proposed to be formed by a nucleation and growth process from the liquid phase. In the present study, the microstructural evolution of bulk specimens during phase conversion were investigated by scanning electron microscopy (SEMI, energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) analySeS.
On the synthesis of high-purity 2223 phase of the Bi(Pb)-Sr-Ca-Cu-0 system, the atmosphere of heat-treatment was found to be one of the dominating factors. Many efforts have been devoted to the studies of phase formation in nitrogen, oxygen, argon and in ambient with different oxygen partial pressure as well as in air [ 1 1- 161. A reduced oxygen partial pressure was found to effectively enhance the efficiency of synthesizing 2223 phase [l l-131. Nevertheless, the atmosphere resulting from the volatile Pb-containing phase of the material did not receive much attention. In the present study the effects of Pb atmosphere on the evolution of microstructures for specimens sintered with and without the Pb atmosphere were investigated.
C280 (1997) 187-199
SEM equipped with a EDAXEDS. For SEM/EDS investigation, the same regions of the specimen were examined repeatedly following heat treatments. XRD patterns of the specimen were taken using a Shimazu XD3A diffractometer with Cu Kol radiation. To study the effect of Pb atmosphere on the formation of 2223 phase, specimens were sintered with fresh pellet samples which were presumed to provide a Pb-rich atmosphere during high temperature. The fresh pellet samples, hereinafter referred to the atmosphere-control (AC) pellet, were prepared with the same starting powder as that for the specimens. The specimens were sintered with and without the presence of AC pellets, cumulatively. As the AC pellets were used for sir&ring, the specimens were placed on the top of the AC pellet and with the surface for characterization facing downward. Since the specimens became a little concave at the surface after the previous experiment, changes in the surface morphology owing to contact with AC pellets was avoided. However, the surface of the specimen was placed face upwards for sintering without the presence of AC pellets. The changes of microstructures, composition, XRD pattern, diameter and weight of specimens were recorded.
2. Experimental 3. Results and discussion The precursor powder with a nominal composition of Bi,,,Pb,.,Sr,,Ca,,Cu,,O, was prepared by a co-precipitation method which was described elsewhere [17]. The calcination of powder was conducted at 760°C for 20 h in ambient atmosphere. Pellet samples with dimensions of 12 mm diameter and N 1.5 mm thickness were prepared by diepressing the calcined powder at 200 MPa. The sintering of specimens was conducted in a tube furnace at 850°C with a heating rate of 180”C/h. The specimens were heat-treated in an ambient atmosphere without gas flowing. After being held at 850°C for a certain period of time, the specimens were cooled down to 800°C at 180”C/h and then quenched in air by quickly being removed from the furnace. The sintering process was repeated until the total sintering time reached 30 h. After each sir&ring process, weight and diameter of specimens were measured. Microstructures were characterized by a CamScan
3.1. Microstructural evolution with time at 850°C Since the Bi(Pb)-Sr-Ca-Cu-0 material contains up to five cations, the phase relations for the formation of desired 2223 phase are known to be very complicated. To investigate the reaction route of phase formation, the characterization of microstructures was often carried out for a series of specimens, e.g. samples sintered at various temperature or time, by XRD, SEM as well as TEM. However, the interaction among grains of various compositions was not directly revealed. As a result, the present experiment was carried out to study the microstructural evolution at a fixed region for the samples heat-treated at a specific temperature for different periods of time. It was aimed to elucidate the interactions among different phases with direct evidence during phase conversion in forming the 2223 superconducting phase.
Y.T. Huang, LJ. Chen/Physica
C280 (1997) 187-199
189
3.1 .I. XRD analysis
To reveal the phase evolution with respect to the sintering time, parts of XRD patterns corresponding to the specimen heat-treated for different periods of time are shown in Fig. 1. Characteristic peaks corresponding to different phases were marked on various patterns for clarity. The major constituents contained in the starting powder were identified to be 2212, Ca,PbO, and CuO phases. After one hour sintering, the corresponding peaks of CuO phase disappeared. The amount of Ca,PbO, decreased as the cumulative sintering time (hereinafter denoted as t,) increased and became undetectable as tf > 4 h. The 2212 phase remained and was the most prominent phase after 4 h sintering. The 2223 phase emerged at t, = 4 h as inferred by the appearance of the diffraction peaks of (0010) and (115) at 28= 23.9” and 26.2”, respectively, The intensities of diffraction peaks corresponding to the 2223 phase become stronger as tc increased. The peaks revealing the simultaneous presence of 2212 and 2223 phases are marked on the pattern for t, = 20 h. Since peaks corresponding to the two phases are appreciably
powder *
15
20
25
30
35
40
45
50
55
62 8
0.5 -
&,
0.4..
$
0.3 -
l**
l
l
l
l l
F 9
0.2 -
0” z
0.1 -
+ 0.0
**
0
5
IO
Cumulative
I5
20
25
30
sintering time, TV(h)
Fig. 2. XRD peak intensity ratio, H(oO~)/H(oO~) specimen at various sintcring time.
+ L@O8),of a
overlapped, it is convenient to use the change of peak-intensity ratio of H(OO10)/H(OO10) + L(OO8) to represent the progress of ph%e convezion. H and L stand for the diffraction peaks corresponding to the 2223 and 2212 phases, respectively. As shown in Fig. 2, the phase conversion was quite rapid at first and slowed down for tc longer than 10 h. The change of conversion rate at long tc will be discussed later. It is well documented [5,9,18-201 that the Ca,PbO, is one of the key constituents to result in a liquid phase which accelerates the growth of 2223 phase in the Bi(Pb)-Sr-Ca-Cu-0 system. As revealed in Fig. 1, the 2223 phase was absent from the XRD pattern in specimens with t, up to 2 h. During the same period, Ca,PbO, and CuO diminished significantly with time. There is a time lag between the dissolution of Ca,PbO, phase and the formation of 2223 phase. It is attributed to the small quantity of the formed 2223 phase that is difficult to be detected by XRD. The peaks corresponding to the Ca,PbO, phase disappeared from XRD patterns for tc > 4 h. However, the phase conversion still proceeded. It is suggested that most of the Ca,PbO, dissolves into a liquid phase and acts as a source to provide Ca and Pb atoms in growing the 2223 phase.
20 degree (CuKa) Fig. 1. XRD patterns of starting powder and a specimen sintered with various cumulative time. 0: 2212, 0: 2223, *: Ca,PbO,, V: CuO, l : (Ca,Sr),CuO,, 0: (Ca,Sr),,Cu,,O,,.
3.1.2. Weight loss and diameter change Fig. 3 shows the weight and diameter changes of the specimens with respect to the sintering time. The
Y.T. Huang. LJ. Chen/Physica
weight of specimens decreased significantly during the first hour. It is attributed mainly to the removal of moisture absorbed in the starting powder. As f, increased the weight loss continued but became sluggish. Evaporation of specific chemical species such as PbO is proposed to be responsible for the weight loss, and/or the change of oxygen stoichiometry when Pb+4 reduced to Pb+’ as it was incorporated into 2223 phase from the parent phase of Ca,PbO, [21]. The diameter of the specimen also showed a great shrinkage during the first hour. It is believed that the formation of liquid phase by dissolving 2212, Ca,PbO,, CuO and/or other intermediate phases is responsible for this dimensional change. In the initial stage of reaction, the constituents responsible for the occurrence of liquid phase are finely dispersed in the powder and the formation of liquid phase is therefore prominent. The great shrinkage of specimen can be expected due to the elimination of porosity and rearrangement of grains whenever abundant liquid phase is present. However, the specimen started to swell as tc exceeded 2 h and continued as t, increased. As predicted, if the reaction follows a liquid-phase-sintering mechanism [22], the specimen swells if that liquid phase is absorbed by the solid phase. It is consistent with our previous suggestion that the 2223 phase nucleated and grew up from the liquid phase [lo]. The starting constituents of 2212, Ca,PbO, and CuO dissolved into a liquid phase which provided the chemical species for growing 2223 phase. The 2223 grains grew up anisotropically with plate-like shape and left a lot of voids among them. It is the dominating mechanism of the diameter change for r, > 2 h. 3.1.3. SEM/ EDS analyses Characteristic microstructural evolution of surface morphology with sintering time is shown in Fig. 4. Since the back-scattered electron image can reveal compositional contrast of impurity phases and their distribution, it was chosen for obtaining the microstructural images. Fig. 4(a) shows the starting microstructure prior to the heat treatment where the sizes of most grains ranged from 0.3 to 1.0 km. The grains exhibited three typical contrasts of black, dark-gray and bright. The areas with black contrast correspond to the locations of pores. The compositions of specific areas or grains were determined by
C280 (1997) 187-199
EDS. Because the typical sampling depth of EDS is about 1 km, it is difficult to determine precisely the composition of individual small grains. However, the dark-gray phase was always found to contain much more Cu and Ca elements than the bright phase. For some selected large grains or agglomerates (m 1 pm> of bright phase, the average composition was determined with the cation ratio of Bi:Pb:Sr:Ca:Cu = 1.940.11: 1.95:0.99:2.0. The ratio corresponds closely to the 2212 phase. The surface morphology changed drastically for specimens subjected to heat-treatment for 1 h. The compositional contrast between the bright and darkgray phases became more evident. The fraction of the dark-gray phase is about 10% as estimated from the micrograph. As r, increased, large plate-like grains were found in specimens with r, = 4 h. Some of the large plate-like grains viewed edge-on are seen clearly in Fig. 4(d). Compositional analysis by EDS showed that they correspond to the 2223 phase. By comparing the same region in micrographs corresponding to fc = 2 and 4 h, most of the observed 2223 grains in specimen with rc = 4 h were found to grow from those embryos found in specimens with tc = 2 h. This indicated that the 2223 grains were mostly nucleated prior to f, = 2 h. However, the presence of 2223 phase was still undetected by XRD owing to its small quantity in specimen with tc = 2 h. For t, = 4 h, the anisotropy of 2223 grains became such that the aspect ratio of broad face to
0.0
8
0
&
0
$$ -1.6 00 . & -1.7 8 9 -*.* ..6”On
0
00 0
%j :;:; :
00 0
3
iii 2 8
“4 6 .
0
-2.2 :
,g
. -0
0
qo0
0 -2.1 -
o”
.
-4
Cl
CD
_ 4 ,i? 0 -
000 - -12
-2.3
’ 0
.
’ 5
.
’ 10
’ I5
.
’ 20
.
’ 25
.
’ 30
Cumulative sintering time, t:(h) Fig. 3. Weight aud diameter changes of a specimen with respect to cumulative sinteriug time.
,lOpm, Fig. 4. Surfam morphology
of a specimen cumulatively
sintered for (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 8 h, (0
192
Y.T. Huang, LJ. Chen/Physica
Table 1 Compositional analyses of specific phases and starting powder by EDS and ICP-AES, respectively
Bi 2212 2223 1424’ 2:lb Ca, PbO, Bullc(1, = 4 h) Powder(by ICP-ASS)
Pb
Sr
24.5 2.9 21.8 21.9 2.9 19.2 1.8 0.3 13.6 3.7 0.7 6.5 9.8 7.8 22.9 17.9 2.5 17.9 17.80 4.22 19.44
Ca
Cu
15.7 23.1 21.9 54.3 48.6 24.8 23.59
33.3 33.2 62.5 34.7 10.9 36.7 34.95
4b Alkaline earth cuperateswith fonuula of (Ca,Sr, _x),4Cu2404, aud (Ca,Sr, _,),CuO,, respectively. can reach 30. The typical thickness of grains is less than 0.3 pm as observed for grains viewed edge-on. The broad face of grain is commonly known to correspond to the ub plane. Compositional analysis of specific grains in specimen with f, = 2 and 4 h was conducted by EDS. The results are listed in Table 1. The small flaky grains with bright contrast correspond to the 2212 phase, and the thin plate-like grains with similar contrast but much larger aspect ratio correspond to the 2223 phase. In addition to the distinct appearance, the Sr/Ca ratio and concentration of Bi were always lower in the 2223 phase than those in the 2212 phase. For 2212 phase the Sr/Ca ratios ranged from 1.3 to 1.5, whereas they were 0.8 - 0.9 for the 2223 phase. The Bi concentration was found to be 23.5 25.5 at% in the 2212 phase and usually less than 22.0 at% in the 2223 phase. It is worthwhile to note that the Pb concentration is higher and the Sr/Ca ratio is lower for the 2212 grains in the heat-treated specimens than those in the as-pressed specimens as mentioned previously. This may be attributed to the existence of a thin layer of liquid phase rich in Pb, Ca and Cu atoms which formed on the surface of these grains during phase conversion. The TEM study of amorphous layers between grains was previously reported [ 19,231. thickness
ent
For the grains with dark-gray phases were identified.
contrast, two differThe first one is
(Ca,Sr,_X),,Cu2404, with x - 0.6 and the other is (Car&, _,),CuOs with y - 0.9, hereinafter denoted as 14:24 and 2:l phase, respectively. These two phases are similar in appearance but with a slight difference in contrast. The 2:l phase is of darker appearance than the other one, which is consistent
C 280 (1997) 187-199
with the fact that back-scattered electrons are lower
in intensity for the smaller mean atomic number in the 2:l phase. Since these two dark-gray phases are relatively small in quantity and the corresponding XRD peaks overlapped with those of 2212 phase, it was difficult to identify them unambiguously from XRD patterns. For the XRD pattern with fc = 2 h, as shown in Fig. 1, the first three strongest peaks belonging to the two dark-gray phases are marked following that reported by Reardon and Hubbard [24]. However, only the peak of 2: 1 phase at 28 36.3” can be unambiguously identified when compared with the peaks of 2212 phase shown above. In Fig. 4(b)-(h), most of the dark-gray grains were firstly found in the specimens with rc = 1 h, i.e. in Fig. 4(b). The grain sizes of dark-gray phases are about several pm as seen in Fig. 4(b), which is evidently larger than the average grain size of starting powder as shown in Fig. 4(a). It is suggested that these large dark-gray phases are formed by the phase-conversion reaction. This is consistent with XRD results, in which no corresponding peaks of both 1424 and 2:l phases were present in the starting powder but they showed up in specimens for r, > 1 h. The fifth phase seen from the image with r, = 4 h are sphere-like grains, bright in contrast, and 0.3 - 0.5 pm in diameter. It is difficult to determine the exact composition by EDS. However, these grains always possess a unique Ca/Pb ratio close to 2/l and much higher Pb concentration with respect to other phases. They are thus inferred to be the Ca,PbO, grains. Typical compositions of this species are listed in Table 1. The average compositions of large areas (120 pm X 90 pm) for the specimens with t, = 4 h determined by EDS and starting powder analyzed by an inductively coupled plasmaatomic emission spectroscopy (ICP-AES) method are also listed in Table 1 for comparison. For t, exceeding 8 h, some 2223 grains grew to impinge one another. For f, = 12 h, most of the 2223 grains were linked together and stopped growing. Since the 2223 phase grains tend to grow anisotropitally or edgewise, the impediment of growing front slowed down the growth rate of 2223 phase. Nevertheless, small new 2223 grains emerged and grew up quickly before they touched neighboring grains of the same phase. During the phase conversion, the flaky grains of 2212 phase were always found to
193
Y.T. Huang, LJ. Chen/ Physica C 280 (1997) 187-199
decrease in size with f,. This indicated that the dominating mechanism of forming the 2223 phase is not the intercalation mechanism for our specimens; otherwise, the 2212 grains would instead grow larger and increase concentrations of both Ca and Cu elements. The compositions of specific 2212 grains analyzed by EDS showed no significant change with increasing t,. For the specific circled area A in r, = 8 h micrograph, the large grain at the bottom is a 2223 grain. The small bright grains distributed on it are of 2212 phase. The dark-gray grain on left side is a 1424 grain. The needle-like grains present neighboring the bottom grain are also of 2223 phase. The 2212 grains diminished as t, increased. Although they look like being isolated above the 2223 grains in the beginning, they may change their size through surface diffusion in transporting mass during phase conversion. The presence of thin-layer liquid phase, observed in previous TEM studies [19,23], along the grain boundary is likely to enhance the mass transport. This scenario is the possible mechanism of phase conversion at long f,. The 14:24 grains also decreased in size but only very slightly. The bottom 2223 grain evidently stretched out downward with increasing r, up to 12 h, but the growth became sluggish as r, increased further, since it was impeded by the surrounding 2223 grains. In the circled area B, there were either two 2223 grains on both sides of a 2:l grain or one large 2223 grain beneath the 2: 1 grain. The 2223 grain finally protruded into the 2: 1 grain as phase conversion proceeded. Similarly, the 2223 grain cut a 2212 grain apart at the circled area C as t, increased. It is suggested that the growth front of the 2223 grain is very active and may dissolve the other phases catalytically. From the present observation, it is clearly shown that the growth front stretched out very quickly when it was not impeded by other growing 2223 grains. The proceeding of the growth front stopped when it was in contact with other grains of the same phase. From these observations, it is believed that the phase conversion of the 2223 is actually rather rapid in contrast to the previous understanding of sluggishness. It is suggested that the highly anisotropic growth results in the sluggishness and difficulty in synthesizing the phase in pure form. Based on this consideration, the efficiency of synthesizing the 2223 phase
O> 0
5
10
15
20
25
30
Cumulative sintering time, k (h) Fig. 5. Chemical composition determined by EDS analysis for specimens at various sintering time.
can be substantially enhanced by taking an intermediate grinding process during the sintering treatment. The grinding process is expected to break up the interconnected 2223 grains and enhance the conversion rate by creating new growth fronts. Fig. 5 shows the compositional change of the examined surface for a large area (120 X 90 pm) with increasing r,. No significant changes in compositions of elements, except Pb, were found. It is clearly seen that the concentration of Pb dropped significantly between tc = 1 and 2 h. This coincides with the change of Ca,PbO, revealed in the XRD patterns shown in Fig. 1. It is attributed to the great dissolution of Ca2Pb0, in the initial stage of phase conversion which resulted in the substantial loss of Pb by evaporation. Comparing the results shown in Figs. 2 and 3, and the Pb loss in Fig. 5, the results appear to be inconsistent in depicting the phase conversion. The significant Pb-loss for specimens characterized by EDS appeared at t, = 2 h. Significant slowing down of the formation of 2223 phase was found at r, > 10 h as revealed by XRD. The changes of weight and diameter of the specimens were relatively gradual and smooth. The discrepancy is believed to result from different spatial resolution for respective measurements. The diameter and weight changes of specimen represent the overall variation of specimen during phase conversion. The XRD analysis revealed the changes in microstructures within several tens of pm from the surface.
194
Y.T. Huang, LJ. Chen/Physica
The EDS analysis obtained compositional information from the region a few pm in depth. Since the evaporation of Pb is much easier from material near the surface of specimen, there should be a concentration variation along the depth of specimen. The results suggested that the microstructures in the interior of bulk specimens are different from those of the surface. For characterizations conducted at different depths of specimens, different results were obtained. 3.2. Effects of Pb atmosphere on microstructural evolution In the previous part of study, the phase conversion of specimen was found to vary from the surface to the interior for bulk specimen due to the inhomogeneous evaporation of Pb (hereinafter stands for the rich Pb-containing species). In addition, the formation of 2223 phase in the previous part of study as revealed by XRD analysis is much less than expected for total sintering time up to 30 h. The phase conversion rate was found to be much higher for a sample heat-treated with normal sintering up to 30 h, where the sample was heat-treated at a fixed temperature for a certain period without interruption. It was suggested that the significant loss of Pb for the cumulative sintering process may be responsible for the low conversion rate. A study of the Pb atmosphere for the formation of 2223 phase on the microstructural evolution has been carried out. 3.2.1. XRD Analysis To illustrate the differences between the cumulative and the normal sintering, a series of samples for normal sintering were prepared of which each sample was heat-treated at 850°C for a certain period of time. The starting powder of the series of samples is the same as that used for cumulatively sintered specimens. Pellet samples were prepared and sintered at 850°C for 2 N 40 h. Fig. 6 shows the H(0010)/[H(0010) + L(OOS)] ratio with respect to sintezg time fz both processes. It is clearly seen that the phase conversion is much faster for the samples sintered under normal processing for sintering time within 30 h. The possible mechanism to cause this difference will be discussed later. For the specimens sintered cumulatively with the
C 280 (1997) 187-199
1.0 -
s 8 =i
normal
.
sAlterin%
0.8 -
0
o,”
I * I. 10
20
30
q
n
0
,
‘---
0.0 - I$
n
0
Pb-atmosphere
I. 40
I. 50
Sintering time
I. 60
exp.
I.
1. 60
70
I 90
(h)
Fig. 6. XRD peak intensity ratio, H@O~)/H(OO1O)+ L@O8), with respect to sinteriag time for normal and cumulative sinterhg, respectively. Filled symbols denote specimens sintered with AC pellets.
AC pellets for additional two 10-h periods, i.e. t, = 40 and 50 h, the phase conversion was found to be significantly enhanced. The 2212 phase almost disappeared on the XRD pattern with t, = 50 h. Little change was recognized by XRD analysis for higher t, of 60, 70 and 90 h in spite of sintering with or without AC pellet. This indicates that the phase conversion of specimens was nearly completed for t, exceeding 50 h. XRD patterns of specimens at various tC are shown in Fig. 7.
15
20
25
30
35
40
45
50
55
29 degree (CuKa) Fig. 7. XRD patterns of specimen at various t,. Asterisks denote specimens sintered with AC pellets. (0: 22,2, 0~ 2223, +: (CaM,CuOJ.
Y.T. Huang, LJ. Chen/Physica
,
IS - 12
.
0
B
.z
.
”
8 0
0 -2.7 -
0.
00
-2.4 -
3
0
-
.
e-6
0
10
20
30
4U
50
6U
70
80
s --9
Pb-almosphererxp.
-
0
_(D
0.
1
90
-
-12
Cumulative sintering time, tc (h) Fig. 8. Weight and diameter changes of specimens with respect to sintering time. Filled symbols denote specimens sintered with AC pellets.
3.2.2. Diameter and weight changes Fig. 8 shows the changes of diameter and weight for specimens processed by cumulative sintering. The filled and hollow data marks stand for the specimens sintered with and without AC pellets, respectively. The results for t, within 30 h are also presented for comparison. As described in an earlier section, the diameter and weight changes appeared to be saturated for t, approaching 30 h. However, specimen diameter was found to continue increasing for specimens sintered under the effect of Pb atmosphere. The change of diameter reached a saturation region for tc exceeding 50 h. The. results are consistent with those obtained by XRD analysis as discussed earlier. The swelling of specimen was attributed to the same liquid-phase-sintering mechanism. Nevertheless, the weight loss of specimens was found to continue appreciably with t, and did not reach a saturation point. The mechanism of the continued loss in weight is not well understood at this time. 3.2.3. SEM/ EDS characterization The compositional changes in a large area ( w 120 x 90 p,m) of specimens are shown in Fig. 9. The compositional changes of specimens at t, within 30 h are also illustrated for comparison. It is clearly seen that only the concentration of Pb varied significantly in the presence of AC pellets. For specimen sintered with AC pellets, the Pb concentration was found to increase as shown in Fig. 9. Further sinter-
C 280 (1997) 187-199
195
ing without the presence of AC pellet, at tc = 60 h, the Pb concentration decreased significantly. The same results were reproduced for sir&ring with and without the presence of AC pellets for t, = 70 and 90 h, respectively. It is suggested that the Pb-rich species dissipate mostly from AC pellets, in which the Ca,PbO, phase is abundant, in establishing the equilibrium partial pressure of Pb during sintering. As a result, the Pb concentration of specimens increased as Pb species condensed on and/or absorbed by the specimens during cooling period, especially near the surface region of specimens. However, for sintering without the AC pellets, the Pb in specimens evaporated in order to balance the partial pressure during high temperature. Therefore, the Pb concentration was found to decrease in specimens sintered without the presence of AC pellets. The suggested mechanism explains well the difference between the normal sintering and the cumulative sintering in phase conversion rate as shown in Fig. 6 with tc under 30 h. The equilibrium partial pressure of Pb was established in the ambient by evaporating Pb from specimens whenever they were heated up to 850°C and some of Pb was dissipated and condensed onto the wall of the quartz tube as confirmed by the appearance of a layer with light-yellow color. For cumulative sir&ring process, the Pb atmosphere was established and some of Pb dissipated away during each
i0
"'.'.."'.'. 0 IO 20
30
Pb-atmosphereexp. 40
50
I.I.I.1 60 70 80
90
Sintcring time, tc (h) Fig. 9. Chemical composition determined by EDS analysis for specimens at various sintering time. The filled marks denote specimens sintered with AC pellets.
1%
Y.T. Huang, LJ. Chen/Physica
sintering. Therefore, each heating-cooling cycle results in a certain loss of Pb concentration from the specimens. Since Pb-containing phase (e.g. Ca,PbO,) is the key constituent in forming liquid phase, the significant loss of Pb leads to the slow conversion rate of forming 2223 phase. For the normal sintering process, the equilibrium Pb atmosphere was established as the specimen was heated up to the sintering temperature and persisted during the sintering period. Although the dissipation of Pb from the specimen still occurs, the rate is substantially lower compared with that of iterative heating-cooling cy-
C280 (1997) 187-199
cles for cumulative sintering process. As a result, the Pb concentrations in the specimens sintered differently were found to vary. It is worthwhile to mention that, in spite of the difference in phase conversion rate, the species of crystalline phases present in samples of the two processes are the same. The amount and grain size of the phases were found to vary with process. Fig. 10 shows a series of micrographs revealing the evolution of microstructures. The small flaky 2212 grains seen in the micrograph with r, = 30 h diminished significantly as the specimens were sin-
Fig. 10. Surface morphology of specimens at r, = (a) 30 h, (b) 40 h, (c) 50 h, (d) 60 h, (e) 70 h and (f) 90 h.
Y.T. Huang, L.J. Chen/Physica Table 2 Compositional
analyses
of specific grains at various
Grain
r, (h)
Bi
Pb
2:1 a
30 40 ’ 50 ’ 60 70 ’ 90
3.8 2.3 2.4 3.5 3.0 3.2
0.5 5.6 3.8 1.0 8.7 0.5
30 70 c 90
1.8 1.7 1.8
0.1 0.2 0.0
14~24~
Sr
r,
Ca
Cu
6.6 5.2 5.8 6.8 5.5 6.8
54.4 53.9 52.5 52.0 47.7 53.5
34.7 33.0 34.5 36.6 35.1 36.0
13.4 13.0 13.0
21.9 21.9 22.4
62.8 63.2 62.8
‘b Alkaline earth cuperates with formula of (Ca,Sr, _&ZU~~O~, and (Ca,Sr, _,),CuO,, respectively. ’ Specimens sir&ted in the presence of AC pellets.
tered further for 10 h ( f, = 40 h) in the presence of AC pellets. It is consistent with the result of XRD analysis, in which the diffraction peaks of 2212 phase decreased notably at f, = 40 h. As tc increased to 50 h, the small 2212 grains almost completely disappeared as revealed by the corresponding micrographs. The surface morphology did not change markedly as t, was increased further. The observation is consistent with the XRD analysis which reveals that the phase conversion entered a saturation region for rc exceeding 50 h. Slight but significant changes were observed for the dark-gray phases. For the grains marked A in the micrograph with re = 30 h, they were identified to be 2: 1 phase. Their appearance changed into a fractured surface morphology and bright in contrast when they were sintered with AC pellet. However, the same regions were converted back to dark-gray regions with smooth appearance for the specimen sintered without AC pellets at f, = 60 h. Similar results were observed for f, = 70 and 90 h. For the circled 2: 1 phase grain at the middle-left of Fig. lo(a), Table 2 lists the change of chemical composition with respect to sintering time. From compositional analysis, the 2:l phase grain with bright contrast contained higher concentration of Pb than that with dark-gray contrast. The other elements, however, did not vary significantly in comparison. The 2:l phase seems to be less stable in the presence of rich Pb atmosphere when sintered with AC pellets. They interacted with Pb atmosphere and formed a phase with fractured appearance with high Pb concentration. The grains
C 280 (1997) 187-199
197
became smooth in appearance and dark-gray contrast again when they were heat-treated further in the absence of AC pellets. For each cycle of sintering with and without the presence of AC pellets, the grain sixes of 2:l grains were found to decrease significantly. The Pb atmosphere established with the aid of the AC pellets may speed up the dissolution of the 2:l phase and thus provide the source of Ca and Cu elements for further phase conversion of the remaining 2212 grains. The dissolution of the 2:l phase in the presence of the Pb atmosphere may also promote the formation of an additional amount of liquid phase in the specimens and further enhance the phase conversion as observed for the specimens sintered for rc = 40 and 50 h. This mechanism is consistent with the previous suggestion that the significant depletion of Pb concentration in specimen may result in the retardation of phase conversion for cumulative sin&ring process at rc below 30 h. In addition, as shown in Fig. 7, the diffraction peak of 2: 1 phase at 2 0 N 36.3” for specimens with rc = 30 h was found to be absent in the specimens sir&red with fe exceeding 40 h. The B grains were identified to be 1424 phase. For some remaining 14:24 phase grains found at various r,, the chemical compositions determined by EDS are listed in Table 2. It is clearly seen that the Pb concentration was slightly increased and the concentration of other elements maintained the same ratios for specimens sintered with AC pellets. In addition, the 1424 grains as seen in micrographs always showed the same dark-gray contrast despite the presence of AC pellets for sintering. It seems that this phase did not change with the effect of Pb atmosphere. The slight increase of Pb concentration for these grains is likely to result from a simple condensation of Pb-rich phase from the atmosphere for sintering with AC pellet rather than an absorption by chemical reaction. For the grain marked C in the specimen with ?, = 50 h, it changed its appearance from a grain with bright contrast, which is recognized as a 2212 grain, into a rounded grain with dark-gray contrast. This grain did not appear before t, = 40 h. The rounded dark-gray grain was identified to be of a typical 1424 grain. The observation may help to clarify the formation route of 1424 phase grains. During phase conversion, the 2212 grains decom-
Y.T. Huang, LJ. Chen/Physica
posed when reacted with other constituents in forming the liquid phase and the resulted 2223 phase. As the phase conversion reaction proceeds disproportionally, where the other essential reactants for reaction are insufficient to consume the whole 2212 grain, the remaining chemical species are very likely to form the 1424 phase. The present observation supports the earlier suggestion that the 14:24 grain phase may be formed during phase conversion, since the grain size of the dark-gray phase is much larger than that of starting powder. In the same context, the formation of 2:l phase possibly resulted from another disproportional reaction of the constituents in the starting powder such as Ca,PbO,, because the 21 phase is closely related to the presence of Pb atmosphere. The 2:l phase may be generated from the liquid phase, which is rich in Ca, Pb and Cu, by simply depleting Pb. This suggestion is strongly supported by the recent finding by Sung and HellStrom [25], in which the 2:l phase was found to form near the interface of Ca, PbO, + CuO/2212 during phase conversion. The present study indicates that the Ca,PbO, phase decomposes readily at the initial stage of phase conversion and the 2:l phase is not stable in the Pb-rich atmosphere during sit&ring. The 2: 1 phase grains were found to decrease with sintering time and no new 2:l grains were found to form in the areas examined in the present study for f, exceeding 4 h. The impurity phase of more interest in obtaining high-purity 2223 phase is the 1424 phase. It is commonly found in the 2223 bulk and tape-wire materials. This phase may be formed by the decomposition of the 2212 phase at any stage when the disproportional reaction occurs during the phase conversion.
4. Conclusions The interactions among various intermediate phases during phase evolution in forming the 2223 phase were elucidated with SEM/EDS and XRD analyses. The results clearly indicated that the dominating mechanism in forming 2223 phase goes through the nucleation and growth route. The strongly anisotropic growth of 2223 phase grains was sug-
C 280 (1997) 187-199
gested to be responsible for the retardation in synthesizing the 2223 phase in pure form. Evaporation of Pb-containing species from the surface is suggested to result in an inhomogeneous microstructure from the surface to the interior of bulk specimens. By a simple method to vary the condition of Pb atmosphere for sintering, the phase conversion reaction of 2223 phase was found to be significantly varied. For the specimens of which the Pb concentration was significantly depleted, the phase conversion was found to be very sluggish or even stopped. By supplying an appropriate Pb atmosphere, the conversion reaction was found to proceed significantly faster. In addition, the presence of the alkaline-earth cuprate phases was also found to be closely related to the condition of Pb atmosphere during sintering treatment. These results provide important insight in synthesizing high-purity 2223 phase for the Bi(Pb)-Sr-Ca-Cu-0 system. References [l] A.P. Malozemoff, Q. Li, S. Fleshier. G.N. Riley Jr., G. Sbnitchler. D. Aizcd, Chin. J. Phys. 34 (1996) 222. [2] H. Maeda, Y. Tanaka, M. Fukutomi, T. Asano, Jpn. J. Appl. Phys. 27 (1988) L209. [3] M. Takano, J. Takada, K. Gda, H. Kitaguchi, Y. Miura, Y. Ikeda, Y. Tomii. H. Ma&i, Jpn. J. Appl. Phys. 27 (1988) LlO41. [4] H. Nobumasa, K. Shim& Yukishige, Y. Kitano, T. Kawai. Jpn. J. Appl. Phys. 27 (1988) L846. [5] N. Kijima, H. Endo, J. Tsuchiya, A. Sumiyama, M. Mizuno, Y. Oguri. Jpn. J. Appl. Phys. 27 (1988) Ll852. [6] P.E.D. Morgan, R.M. Housley, J.R. Porter, J.J. Ratto, Physica C 176 (1991) 279. [7] M. Wang, Cl. Xiong, X. Tang, Z. Hong, Physica C 210 (1993) 413. [8] W. Bian, Y. Zhu, Y.L. Wang, M. Suenaga, Physica C 248 (1995) 119. [9] Y.T. Huang, W.N. Wang, S.F. Wu, C.Y. Shei, W.M. Humg, W.H. Lee, P.T. Wu, J. Am. Ceram. Sot. 73 (1990) 3507. [lo] Y.T. Huang, C.Y. Shei, W.N. Wang, C.K. Chiang. W.H. Lee, Physica C 169 (1990) 76. [l 11 U. Endo, S. Koyama, T. Kawai, Jpn. J. Appl. Phys. 27 (1988) L1476. [12] S. Koyama, U. Endo, T. Kawai, Jpn. J. Appl. Phys. 27 (1988) Ll861. [13] K. Aoto. H. Hattori, T. Hatano. K. Nakamura, K. Ogawa, Jpn. J. Appl. Phys. 28 (1989) L21%. [14] N.M. Hwang, G.W. Bahng, H.G. Moon, J.C. Park, Appl. Phys. Lett. 54 (1989) 1588. [15] Y.L. Chen, R. Stevens, J. Am. Ceram. Sot. 75 (1992) 1160. [16] W. Zhu, P.S. Nicholson, J. Appl. Phys. 73 (1993) 8423.
Y.T. Huang. LJ. Chen/Physica [17] D.H. Chen. C.Y. Shei. S.R. Sheen, CT. Chang, Jpn. J. Appl. Phys. 30 (1991) 1198. 1181 T. Uzumaki, K. Yamanaka, N. Kamehara, K. Niwa, Jpn. J. Appl. Phys. 28 (1989) L75. [19] Y.L. Chen, R. Stevens, J. Am. Ceram. Sot. 75 (1992) 1142. [20] W. Wang-Ng, C.K. Chiang, S.W. F&man, L.P. Cook, M.D. Hill, Am. Ceram. Sot. Bull. 71 (1992) 1261.
C 280 (1997) 187-199
199
[21] M.G. Smith, D.S. Phillips, D.E. Peterson, J.O. Willis, Physica C 224 (1994) 168. [221 R.M. German, Liquid Phase Sintering, ch. 4, Plenum Press, New York, 1985. p. 65. [231 H.K. Liu, R.K. Wang, S.X. Dou, Physica C 229 (1994) 39. [24] B.J. Reardon, G.R. Hubbard, Pow. Diff. 7 (1992) 142. [25] Y.S. Sung, E.E. Hellstrom, Physica C 253 (1995) 79.